Selective dimethylether and methanol recovery and recycle in a methanol-to-olefin process

ABSTRACT

An improved methanol-to-olefin (MTO) product recovery scheme is provided which enables substantial reduction in the amount of light olefins that are undesirably captured in a dimethylether (DME) recycle stream and improved recovery of methanol when a portion of the methanolic feed to the MTO reaction zone is used as the scrubbing solvent in a primary DME absorption zone in order to recycle this DME oxygenate by-product to the MTO reaction zone. In accordance with the present invention, a liquid solvent stream recovered from the primary DME absorption zone is subjected in a stripping zone to light olefin stripping conditions sufficient to lift a substantial portion of the light olefins that are absorbed in the DME solvent without stripping a significant portion of this methanol solvent, thereby increasing the recovery of desired light olefins while simultaneously diminishing the amount of light olefins carried by the DME recycle stream back to MTO conversion step.

FIELD OF THE INVENTION

The present invention relates to the selective recovery and recycle ofdimethylether (DME) and methanol from the effluent stream from amethanol-to-olefin (MTO) catalytic conversion process. The presentinvention relates more specifically to control of the undesired C₂ andC₃ olefin content of the principal DME recycle stream which has beenfound to be a detrimental effect of prior art methods for performingthis recovery and recycle of DME.

BACKGROUND OF THE INVENTION

A major portion of the worldwide petrochemical industry is concernedwith the production of light olefin materials and there subsequent usein the production of numerous important chemical products viapolymerization, oligomerization, alkylation and the like well-knownchemical reactions. Light olefins include ethylene, propylene andmixtures thereof. These light olefins are essential building blocks forthe modern petrochemical and chemical industries. The major source forthese materials in present day refining is the steam cracking ofpetroleum feeds. For various reasons including geographical, economic,political and diminished supply considerations the art has long sought asource other than petroleum for the massive quantities of raw materialsthat are needed to supply the demand for these light olefin materials.In other words, the holy grail of the R & D personnel assigned to workin this area is to find a way to effectively and selectively usealternative feedstocks for this light olefin production applicationthereby lessening dependence of the petrochemical industry on petroleumfeedstocks. A great deal of the prior art's attention has been focusedon the possibility of using hydrocarbon oxygenates and more specificallymethanol as a prime source of the necessary alternative feedstock.Oxygenates are particularly attractive because they can be produced fromsuch widely available materials as coal, natural gas, recycled plastics,various carbon waste streams from industry and various products andby-products from the agricultural industry. The art of making methanolfrom these types of raw materials is well established and typicallyinvolves the use of one or more of the following procedures: (1)manufacture of synthesis gas by any of the known techniques typicallyusing a nickel or cobalt catalyst followed by the well-known methanolsynthesis step using relatively high pressure with a copper-basedcatalyst; (2) selective fermentation of various organic agriculturalproducts and by-products in order to produce oxygenates; or (3) variouscombinations of these techniques.

Given the established and well-known technologies for producingoxygenates from alternative non-petroleum raw materials, the art hasfocused on different procedures for catalytically converting oxygenatessuch as methanol into the desired light olefin products. These lightolefin products that are produced from non-petroleum based raw materialsmust of course be available in quantities and purities such that theyare interchangeable in downstream processing with the materials that arepresently produced using petroleum sources. Although many oxygenateshave been discussed in the prior art, the principal focus of the twomajor routes to produce these desired light olefins has been on methanolconversion technology primarily because of the availability ofcommercially proven methanol synthesis technology. A review of the priorart has revealed essentially two major techniques that are discussed forconversion of methanol to light olefins. The first of these MTOprocesses is based on early German and American work with acatalytically conversion zone containing a zeolitic type of catalystsystem. Representative of the early German work is U.S. Pat. No.4,387,263 which was filed in May of 1982 in the U.S. without a claim forGerman priority. This '263 patent reports on a series of experimentswith methanol conversion techniques using a ZSM-5-type of catalystsystem wherein the problem of DME recycle is a major focus of thetechnology disclosed. Although good yields of ethylene and propylenewere reported in this '263 patent, they unfortunately were accompaniedby substantial formation of higher aliphatic and aromatic hydrocarbonswhich the patentees speculated might be useful as an engine fuel andspecifically as a gasoline-type of material. In order to limit theamount of this heavier material that is produced, the patentees of the'263 patent propose to limit conversion to less than 80% of the methanolcharged to the MTO conversion step. This operation at lower conversionlevels necessitated a critical assessment of means for recovering andrecycling not only unreacted methanol but also substantial amounts of aDME intermediate product. The focus then of the '263 patent inventionwas therefore on a DME and methanol scrubbing step utilizing a watersolvent in order to efficiently and effectively recapture the lightolefin value of the unreacted methanol and of the intermediate reactantDME.

This early MTO work with a zeolitic catalyst system was then followed upby the Mobil Oil Company who also investigated the use of a zeoliticcatalyst system like ZSM-5 for purposes of making light olefins. U.S.Pat. No. 4,587,373 is representative of Mobil's early work and itacknowledged and distinguished the German contribution to this zeoliticcatalyst based MTO route to light olefins. The inventor of the '373patent made two significant contributions to this zeolitic MTO route thefirst of which involved recognition that a commercial plant would haveto operate at pressure substantially above the preferred range that theGerman workers in this field had suggested in order to make thecommercial equipment of reasonable size when commercial mass flow ratesare desired. The '373 patent recognized that as you move to higherpressure for the zeolitic MTO route in order to control the size of theequipment needed for commercial plant there is a substantial additionalloss of DME that was not considered in the German work. This additionalloss is caused by dissolution of substantial quantities of DME in theheavy hydrocarbon oil by-product recovered from the liquid hydrocarbonstream withdrawn from the primary separator. The other significantcontribution of the '373 patent is manifest from inspection of the flowscheme presented in FIG. 2 which prominently features a portion of themethanol feed being diverted to the DME absorption zone in order to takeadvantage of the fact that there exist a high affinity between methanoland DME thereby downsizing the size of the scrubbing zone requiredrelative to the scrubbing zone utilizing plain water that was suggestedby the earlier German work.

Primarily because of an inability of this zeolitic MTO route to controlthe amounts of undesired C₄ ⁺ hydrocarbon products produced by the ZSM-5type of catalyst system, the art soon developed a second MTO conversiontechnology based on the use of a non-zeolitic molecular sieve catalyticmaterial. This branch of the MTO art is perhaps best illustrated byreference to UOP's extensive work in this area as reported in numerouspatents of which U.S. Pat. No. 5,095,163, U.S. Pat. No. 5,126,308 andU.S. Pat. No. 5,191,141 are representative. This second approach to MTOconversion technology was primarily based on using a catalyst systemcomprising a silicoaluminophosphate molecular sieve (SAPO) with a strongpreference for a SAPO species that is known as SAPO-34. This SAPO-34material was found to have a very high selectivity for light olefinswith a methanol feedstock and consequently very low selectivities forthe undesired corresponding light paraffins and the heavier materials.This SAPO catalyzed MTO approach is known to have at least the followingadvantages relative to the zeolitic catalyst route to light olefins: (1)greater yields of light olefins at equal quantities of methanolconverted; (2) capability of direct recovery of polymer grade ethyleneand propylene without the necessity of the use of extraordinary physicalseparation steps to separate ethylene and propylene from theircorresponding paraffin analogs; (3) sharply limited production ofby-products such as stabilized gasoline; (4) flexibility to adjust theproduct ethylene-to-propylene weight ratios over the range of 1.5:1 to0.75:1 by minimal adjustment of the MTO conversion conditions; and (5)significantly less coke make in the MTO conversion zone relative to thatis experienced with the zeolitic catalyst system.

Despite the promising developments associated with the SAPO catalyzedMTO route to light olefins, the problem of DME co-production is commonto both types of catalytic MTO routes discussed above and variousmeasures have been suggested in the prior art to recover and recycle DMEfrom the effluent stream from an MTO conversion zone. In U.S. Pat. No.4,387,263, a relatively high pressure DME absorption zone is taughtutilizing a plain water solvent in order to recapture and recycle theDME intermediate. By the use of the term “high pressure” with referenceto this '263 patent, it is pointed out that the examples 1, 2, 3 and 4were run at 2000 kPa (290 psi) and the fifth example was run at an evenhigher pressure of 4000 kPa (580 psi). One of the improvements suggestedby U.S. Pat. No. 4,587,373 focused on utilizing a more efficient DMEsolvent in the DME absorption zone and recommended that a portion of themethanolic feed to the MTO conversion reactor be diverted to the DMEabsorption zone in order to more efficiently recapture the DMEcontaminant from the olefin product stream. As explained above, this'373 patent proposed to reduce commercial plant size by operating theMTO conversion reactor at a much higher preferred pressure than wassuggested by the prior art and specifically focused on a reactoroperation at about 550 kpa (80 psi) but noted that operating the MTOreactor at this high pressure would open the door to substantial DMEloss in the heavy hydrocarbon product stream recovered from theprincipal separator in the effluent work-up portion of the flow schemeunless steps were taken to strip dissolved DME from the heavyhydrocarbon by-product stream. In particular, in the flow scheme of FIG.2 of the '373 patent, it is proposed to strip the heavy hydrocarbonby-product recovered from the primary separator 16 in stabilizer tower26 in order to recapture the value of the DME intermediate dissolvedtherein while simultaneously using a methanol solvent in the DMEabsorber 22.

In my attempts to practice a product recovery flow scheme quite similarto that disclosed in FIG. 2 of the '373 patent in conjunction with theuse of a SAPO-type catalytic system in a MTO conversion zone, I have nowfound a further problem associated with the use of this flow scheme inorder to recapture and recycle the DME intermediate that contaminantsthe effluent stream from the MTO reaction zone. I have found morespecifically that if a portion of the methanol feed to the MTOconversion zone is diverted to the DME absorber as suggested in the '373patent in order to recover DME more efficiently, there is substantialco-absorption of light olefins into the methanol solvent associated withthis scheme. A methanol solvent's ability to extract from the lightolefin-containing input stream to the DME absorber not only DME butsubstantial quantities of C₂ and C₃ olefins was not reported in the '373patent and greatly complicates the design of an efficient productwork-up flow scheme for a SAPO based MTO conversion zone. For example,when the DME absorption zone is operated with a methanol solvent atscrubbing conditions including a temperature of about 54° C. (129° F.)and a pressure of about 2020 kPa (293 psi) with a 99.85 mass-% methanolsolvent, at least 12.3 mass-% of the C₂ olefins and 40.3 mass-% of theC₃ olefins charged to the DME scrubber will be co-absorbed in theDME-rich liquid solvent bottom stream withdrawn from the scrubber. Whenthis DME-rich solvent stream is recycled to the MTO conversion zone, asubstantial internal circuit of light olefins is created which acts tosubstantially increase the size of the MTO conversion zone whileincreasing the rate of detrimental coking on the catalyst containedtherein due to the fact that these C₂ and C₃ olefins are reactive andcan undergo polymerization and condensation to form coke precursors.

The problem addressed by the present invention is therefore tosubstantially diminish this undesired buildup of C₂ and C₃ olefins inthe DME recycle stream flowing to the MTO conversion zone when methanolis used as a solvent in a DME absorption zone that is a prominentfeature of an MTO reactor effluent work-up scheme.

SUMMARY OF THE INVENTION

The primary object of the present invention is to make the recovery andrecycle of DME, methanol and/or other oxygenates found in an effluentstream from an MTO conversion zone more selective relative to the DMErecovery technology taught in the prior art, thereby enabling areduction in size of the MTO conversion zone due to the decrease in theamount of the DME recycle stream. A secondary objective is to provide aselective method of recovery and recycle of the DME intermediate foundin the effluent stream from an MTO conversion zone which selectivemethod substantially diminishes the risk of causing substantial cokingon the catalyst system used in the MTO conversion zone due to internalrecycle of relatively large amounts of very reactive light olefins anddienes.

This problem of light olefin contamination of the methanolic solventrecycle stream is addressed by the present invention by providing aspecially designed stripping zone that is applied to the rich solventstream recovered from the primary DME absorption zone in order toselectively remove substantial quantities of light olefins therefromwithout significantly affecting the methanol content thereof, therebysubstantially diminishing the risks of adverse effects on theperformance of the MTO conversion zones when it is operated in anintegrated manner with a reactor effluent product work-up flow scheme.

I have now found that the problem of light olefin contamination of theprincipal oxygenate recycle stream containing both DME and methanol thatis a prominent feature of the MTO product work-up flow scheme that istaught in U.S. Pat. No. 4,587,373 can be efficiently solved by adding aspecial purpose light olefin stripper to the flow scheme of the '373patent. This stripper operates on the liquid solvent bottom stream fromthe DME absorber and is designed to run at a severity such that asubstantial portion of the light olefins dissolved in this liquid streamare stripped therefrom without lifting a major portion of the oxygenatessuch as DME and methanol that are contained in this stream. The flowscheme of the '373 patent suggests that this oxygenate-rich liquidstream should be subjected to an oxygenate stripping step which isdesigned to lift essentially all of the oxygenates contained therein.This of course differs sharply from the solution embedded in myinvention which is based on the premise that these light olefins can bestripped selectively if the stripping severity level is adjusted so thatit is not severe enough to lift a significant portion of the methanolcontained in this liquid solvent stream but is at a level which willlift substantially 90 to 100% of the ethylene dissolved in this solventstream and approximately 40 to 70% of the propylene.

The present invention is therefore a novel method of selective recoveryof a DME-containing recycle stream and a DME-lean, methanol-lean and alight olefin-rich product stream from the effluent stream from a MTOconversion zone which effluent stream contains by-product water,unreacted methanol, a DME intermediate reactant, ethylene, propylene, C₄to C₆ olefins, and minor amounts of other hydrocarbons and oxygenates.In accordance with a first embodiment of the present invention, thefirst step of this method involves cooling and separating at least aportion of this effluent stream into an aqueous liquid stream containingmethanol and DME, a hydrocarbon liquid stream containing methanol, DME,and C₂ to C₆ olefins and a hydrocarbon vapor stream containing DME,methanol, ethylene and propylene. In the second step of this method, DMEis stripped from the liquid hydrocarbon stream recovered in the firststep in a DME stripping zone operated at stripping conditions effectiveto produce an overhead vapor stream containing DME, methanol, ethyleneand propylene and a liquid hydrocarbon bottom stream containing C₄ to C₆olefins. In the next steps of the instant method, the hydrocarbon vaporstream separated in the first step is combined with at least a portionof the overhead vapor stream produced in the DME stripping step to forma DME-rich light hydrocarbon vapor stream which is then charged to aprimary DME absorption zone and therein countercurrently contacted witha DME selective solvent containing methanol at scrubbing conditionseffective to produce: (1) a liquid solvent bottom stream containingmethanol, DME, water and substantial and undesired amounts of ethyleneand propylene (2) a light olefin-rich, DME-lean overhead vapor streamcontaining methanol. At least a portion of the liquid solvent bottomstream recovered from the DME scrubbing step is then passed to a lightolefin stripping zone operating at stripping conditions effective tostrip at least a substantial portion of the ethylene and propylenecontained in this solvent stream without stripping any significantportion of the methanol therefrom to produce a stripper overhead streamrich in ethylene and propylene and containing trace amounts of DME and aliquid solvent bottom stream containing DME, methanol, water and reducedamounts of light olefin relative to the light olefin content of theliquid solvent bottom stream recovered from the primary DME scrubbingstep. At least a portion of the liquid solvent bottom stream recoveredfrom the light olefin stripping step is then recycled to the MTOconversion zone thereby selectively providing additional oxygenatereactants thereto. In the last step at least a portion of the overheadvapor stream produced in the primary DME absorption zone and at least aportion of the stripper overhead stream from the light olefin strippingzone are charged to a secondary DME absorption zone wherein these vaporstreams are countercurrently contacted with an aqueous solvent atscrubbing conditions selected to produce a DME-lean and methanol-leanoverhead vapor product stream containing ethylene and propylene and abottom stream containing DME, methanol and the aqueous solvent.

A second embodiment involves the method of the first embodiment whereinat least a portion of the aqueous stream separated in the first step iscombined with at least a portion of the bottom stream from the secondaryDME absorption zone and charged to an oxygenate recovery zone operatedat oxygenate stripping conditions effective to produce an overhead vaporstream rich in methanol and DME and an aqueous bottom stream which issubstantially free of oxygenates and wherein at least a portion of thisoverhead vapor stream is recycled to the MTO conversion zone therebyproviding additional oxygentate reactants thereto.

Yet another embodiment of the instant invention involves a furthermodification to the selective DME and methanol recovery methodsdescribed above in the first or second embodiments wherein the strippingconditions utilized in the light olefin stripping step include aseverity level sufficient to produce a liquid solvent bottom streamcontaining less than 1 mass-% ethylene.

A further embodiment involves a modification of the DME and methanolrecovery method described above is the second embodiment wherein theaqueous solvent stream charged to the secondary DME absorption zone isat least a portion of the aqueous bottom stream produced in theoxygenate recovery zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a preferred embodiment of thepresent invention, which portrays the essential interconnections andinterrelationships between the operating zones associated with theinstant selective DME and methanol recovery method.

FIG. 2 is a process flow diagram for a DME recovery scheme of the priorart. Not shown in these drawings are items of equipment that are wellknown to those of ordinary skill in the art such as heaters, coolers,heat exchangers, pumps, compressors, suction drums, knock out pots,condensers, overhead or bottom receivers, controls, valves, reboilersand the like.

TERMS AND CONDITIONS DEFINITIONS

The following terms and conditions are used in the present specificationwith the following meanings: (1) a “portion” of a stream means either analiquot part that has the same composition as the whole stream or a partthat is obtained by eliminating a readily separable component therefrom(e.g. if the stream contains hydrocarbons in admixture with steam, thenafter condensation of a major portion of the steam, it comprises anaqueous portion and a hydrocarbon portion); (2) an “overhead” streammeans the net overhead recovered from the specified zone after recycleof any portion to the zone for reflux or any other reason; (3) a“bottom” stream means the net bottom stream from the specified zoneobtained after recycle of any portion for purposes of reheating and/orreboiling and/or after any phase separation; (4) a line is “blocked-off”when it contains a valve that is set to a position that prevents flowthrough the line; (5) presence of necessary compressors and/or pumps isunderstood when flow is shown from a zone of relatively low pressure toa zone of higher pressure; (6) presence of necessary heating and/orcooling means is implied when flow is shown between zones operating atdifferent temperatures; (7) an ingredient is “lifted” or “stripped” whenit is concentrated in the overhead stream withdrawn from the specifiedzone; (8) a “vapor” stream means a stream containing one or morecomponents in the gaseous state; (9) the term “light olefins” meansethylene, propylene and mixtures thereof and (10) the term “theoreticalplates” as used herein means to the number of contacting stages requiredfor a given mass transfer separation that is obtained utilizing theMcCabe-Thiele assumptions and solution method see Perry, John H.Chemical Engineer's Handbook, 4^(th) Ed (New York: McGraw-Hill, 1963),p. 13–22.

DETAILED DESCRIPTION OF THE INVENTION

The starting point for the present invention is a MTO conversion stepwhich utilizes methanol as the principal source of the oxygenatereactant. As explained hereinbefore, there are essentially two differentapproaches to the catalytic conversion of methanol to light olefins. Theprincipal distinction between these two approaches is based on the typeof molecular sieve which is used as the active ingredient in the MTOcatalyst system and I prefer the non-zeolitic route to MTO conversion.The details associated with this non-zeolitic route to MTO conversionare summarized in U.S. Pat. No. 5,095,163, U.S. Pat. No. 5,126,308 andU.S. Pat. No. 5,191,141, all of the teachings of which are allspecifically incorporated herein by reference. As indicated in theseteachings, the preferred molecular sieve is a silicoaluminophosphatesystem which has been established as occurring in numerous specificcrystal structures. As is indicated in the cited patents, the mostpreferred SAPO structure for MTO conversion has been identified as aSAPO-34 structure. Although the selective recovery method of the presentinvention will work equally well with effluent streams from MTOconversion zones that contain zeolitic or non-zeolitic catalyst systems,it is preferred that the effluent stream be derived from an MTOconversion zone that is run with a SAPO-34 catalyst system. TheSAPO-34molecular sieve maybe used alone or may be mixed with a binderand/or filler and formed into shapes such as extrudates, pills, spheres,and the like. Any of the inorganic oxide well known in the art maybeused as a binder and/or filler such as alumina, silica,alumina-phosphate, silica-alumina, and/or one of the various silica-richclays that are well known to those of ordinary skill in the art. When abinder and/or filler is used in formulating the SAPO-34 catalyst system,SAPO-34 will usually be present in an amount of about 5 to 90 mass-% ofthe finished catalyst and preferably about 5 to 40 mass-% thereof. It isto be understood that the active ingredient is the SAPO-34 molecularsieve and the binder and/or filler is an inert material that is used toprovide structural integrity to the catalyst particles. Best practicewith a SAPO-34 catalyst system is to utilize it in a particle sizesuitable for a fluidized reactor system—typically an average particlesize of 65 to 85 microns.

Although the MTO conversion zone can be operated with any of the reactorconfigurations known to the art, it is preferred to use a dynamic bedsystem instead of a fixed bed system in order to efficiently contact themethanol feed stream with the catalyst particles and facilitate theregeneration of the resulting coked catalyst. Either a moving bed systemor a fluidized bed system may be used with good results. Best practiceis however to use a fluidized bed catalyst system.

The fluidized MTO reaction zone is operated at conditions, which includea temperature of about 300° to 600° C. (572° to 1112° F.) with thepreferred range being about 450° to 550° C. (842° to 1022° F.). Thepressure used in the MTO conversion step is typically in the range ofabout 138 to 1000 kPa (20 to 145 psi) and preferably from about 170 to345 kPa (24.7 to 50 psi). The contact time of the reactants with thecatalyst is ordinarily measured in terms of a Weight Hourly SpaceVelocity (WHSV) calculated on the basis of a mass hourly flow rate ofthe sum of the mass of the methanol reactant passed to the MTOconversion zone, any other oxygenate reactants present in the feed orrecycle and any hydrocarbon materials present therein divided by themass of the SAPO-34 molecular sieve present in the MTO conversion zone.WHSV for use in the MTO conversion zone associated with the presentinvention can range from 1 to about 100 hr⁻¹, with the best resultsobtained in the range of about 5 to 20 hr⁻¹. Since the MTO conversionreaction is strongly exothermic, a significant temperature increase willoccur across the reaction zone ordinarily of the magnitude of about 250°to 500° C. (482° to 932° F.). In a fluidized reactor system a catalystcirculation rate between the reactor and the regenerator will be set ata minimum level desired to hold average coke on the circulatinginventory of the preferred SAPO catalyst in a range of about 1 to 20mass-% of the active ingredient in the catalyst and, more preferrably,in the range of about 5 to 17 mass-%.

The regeneration step associated with the MTO conversion step willordinarily use one of the established oxidative techniques for removingthe necessary amount of coke from the catalyst prior to recirculation tothe conversion zone. The primary factor that will establish thecirculation rate between the conversion zone and the regeneration zoneis the equilibrium value of coke on catalyst that it is desired tomaintain in order to obtain the desired conversion level. SAPO-34 basedcatalyst system runs quite successfully at conversion levels of 95% orhigher and results in a coke make of approximately 2 to 5 mass-% ofmethanol equivalents charged to the MTO conversion step. Knowing thecoking rate someone of ordinary skill in the art can then establish acirculation rate to the regenerator based on burning coke at a ratewhich holds the overall average coke level on the circulating catalystsystem used in the MTO conversion zone in the desired range specifiedhereinbefore. In comparison with traditional FCC operation, thecirculation rate for an MTO fluidized conversion zone will be quite lowsince the regenerated catalyst is not needed to supply heat to the MTOreaction zone.

The methanol feedstock that is charged to the MTO conversion step canordinarily be used with a diluent as is taught in the prior artacknowledged and incorporated above; however, best practice is not touse a diluent other than autogenously produced steam. The use of adiluent is beneficial in the sense of controlling the partial pressureof the methanol reactant but is disadvantageous in the sense ofincreasing the volume of the reaction zone and providing additionalmaterial that has to be separated from the products in the recoverysection of the process. When a diluent is present in the MTO conversionstep, it is preferably steam that is derived from the water that is anevitable contaminant of the methanol feed stream as well as of therecycle oxygenate streams. Since in many cases it is desired to charge acrude methanol feed stream containing up to about 20 wt-% water, theremay in fact be substantial diluent that is brought into the system withthe feed stream. In most cases however, it is preferred to run with amethanol feed stream that is 95 to 99.9 mass-% methanol. It is to berecognized that a substantial amounts of a steam diluent will beautogenously generated in the MTO conversion zone due to the fact thatmethanol can be calculated to contain over 56 mass % bound water and dueto the fact that the kinetics of the reaction occurring in the MTOreaction zone are such that the initial formation of DME is extremelyfast and results in the formation of one mol of a steam diluent forevery 2 mols of methanol that react to produce DME.

The effluent stream withdrawn from an MTO conversion zone will thereforecontain substantial amounts a water by-product as well as unreactedmethanol, substantial quantities of DME intermediate, ethylene,propylene, C₄ to C₆ olefins and minor amounts of other hydrocarbons andoxygenates. Typically, with the preferred SAPO-34 catalyst system whenit is run at 97+ conversion levels, approximately 70 to 78% of themethanol equivalent carbon entering the conversion step will beconverted to the desired C₂ and C₃ olefins with 2 to 5% of the carbonconverted to coke and approximately 0.5 to 1% converted to DME. Thelevel of saturated hydrocarbons produced in the MTO conversion step suchas methane, ethane and propane are characteristically held at very lowlevels with a SAPO-34 catalyst and will approximately be 2 to 5% of thecarbon balance.

The effluent stream exiting the MTO reactor will typically be at arelatively high temperature of 350° to 600° C. (662° to 1112° F.) andmust be substantially cooled prior to entering a phase separation zone.Typically, this cooling is done either by heat exchange against the feedmethanol stream or by the use of an aqueous quench stream in a quenchingtower or a combination of both of these techniques. Regardless of theheat exchange technique that is used, it is preferred to substantiallycool and condense at least a substantial portion of the water by-productcontained in the effluent from the conversion zone utilizing a quenchtower operated with a cool quenching medium consisting essentially ofwater and at quenching conditions whereby the effluent stream ispartially condensed with recovery of a substantial portion of the waterby-product of the MTO conversion zone. The quench tower usually operatesat a pressure which is approximately 40 to 95% of the pressuremaintained in the MTO conversion zone and the overhead recovered fromthis quench tower will still contain substantial amounts of water vaporalong with the hydrocarbon and oxygenate products of the synthesisreaction. A preferred two stage quench tower design for this opeartionis shown in my recently issued patent, U.S. Pat. No. 6,403,854, all ofthe teaching of which are hereby incorporated herein by referenece. Itis preferred to take this overhead stream from the quench tower and runit through a series of suction drums and compressors in order to elevateits pressure to a range of about 2000 kPa (290 psi) to 2600 kPa (377psi).

Referring now to the attached FIG. 1 for details of the product recoveryscheme of the present invention, it shows a schematic outline of theinterrelationship and interconnection between the various zones of apreferred embodiment of the present invention. Zone 1 is the MTOconversion zone and it is charged with a methanolic feedstock thatenters the system via line 9. Zone 1 is operated in accordance with theteachings set forth above to produce an effluent stream which exits zone1 via line 12 and is charged to the lower region of quench zone 2wherein it is countercurrently contacted with a cooled circulatingaqueous stream in order to condense a substantial portion of thesignificant water by-product that is produced in the MTO reaction zone.Zone 2 is shown with an aqueous quenching medium being withdrawn vialine 14 and recirculated to the upper region of quench zone 2 afterpassage through a suitable cooling means (not shown). A portion of thecirculating aqueous stream in the quench zone is withdrawn via line 14and passed to oxygenate recovery zone 3 which operates to stripsubstantially all oxygenates that are dissolved in the circulatingaqueous scrubbing medium and to recover and recycle these oxygenates toMTO zone conversion zone 1 via lines 16 and 9. A water stream which issubstantially free of contaminants is withdrawn from zone 3 via line 17and is the principal outlet for the water by-product. The resultingcooled and quenched overhead vapor stream recovered from quench zone 2via line 13 is then compressed by means not shown to a pressure rangepreviously specified and charged to primary product separating zone 4which is operated at approximately 10° to 100° C. (50° to 212° F.) andprefereably about 20 to 66° C. (68° to 151° F.) to produce a three-phaseseparation. The vapor stream recovered from the three-phase separator 4is withdrawn therefrom via line 22 and passed to the lower region ofprimary DME absorption zone 6. The liquid hydrocarbon phase that formsin zone 4 has substantial amounts of DME, light olefins and unreactedmethanol dissolved therein and it is passed via line 25 into strippingzone 5 where it contacts an upflowing vapor stream generated by areboiler (not shown) under stripping conditions effective to remove DMEand light olefins from this hydrocarbon stream and generate an overheadvapor stream recovered by means of line 26 containing DME, ethylene andpropylene along with minor amounts of light saturates that are dissolvedin this relatively heavy hydrocarbon stream. The liquid hydrocarbonstream withdrawn from the bottom of DME stripping zone 5 is passedtherefrom via line 27 and constitutes a heavy hydrocarbon by-productstream of the MTO reaction which essentially consists of C₄, C₅ and C₆Olefins in admixture with a minor amount of C₄ ⁺ saturates. In thepreferred MTO operation with a SAPO-34 catalyst system, this heavyliquid hydrocarbon stream withdrawn via line 27 will compriseapproximately 8 to 18% of the methanol equivalent carbon value chargedto MTO conversion zone 1.

At the intersection of line 26 with line 22, at least a portion of thehydrocarbon vapor stream from separation zone 4 is combined with atleast a portion of the overhead vapor stream produced in the DMEstripping zone 5 to form a DME-rich light hydrocarbon vapor stream whichis charged via line 22 to the lower region of a primary DME absorptionzone 6. Primary DME absorption zone 6 is a conventional liquid-gascontacting zone packed with a suitable material well known to thoseskilled in the art to enhance vapor liquid interaction as the ascendingvapor stream meets a descending liquid stream. Suitable contacting meansare various shaped packing materials that are well known to thoseskilled in the art as well as vapor liquid contacting trays such as thewell-known multi-downcomer trays that are available from UOP. Thesolvent charged to DME scrubbing zone 6 is in accordance with thepresent invention is a portion of the methanolic feedstock that entersthe flow scheme via line 9 and flows via line 10 to the upper region ofabsorption zone 6 wherein it is passed into countercurrent contact withupflowing DME-rich light hydrocarbon vapor stream which is charged toscrubbing zone 6 via line 22. The packing material used in scrubbingzone 6 is typically a Rashig ring material which is inert to the variousactive ingredients charged to scrubbing zone 6 and has a high masstransfer efficiency for promoting interaction between the upflowingvapor stream and the descending methanolic solvent stream. Scrubbingzone 6 is operated at scrubbing conditions effective to produce a liquidsolvent bottom stream containing methanol, DME, water and substantialand undesired amounts of ethylene and propylene and a light olefin-rich,DME-lean and methanol-containing overhead vapor stream. Preferably, thescrubbing conditions encompass a pressure of about 1896 to 2241 kPa (275to 325 psi) and a temperature of about 20° to 66° C. (68° to 151° F.)where the pressure is measured at the top of the column where theoverhead vapor stream is withdrawn via line 19 and the temperature ismeasured at the bottom of the column where the DME-rich solvent iswithdrawn via line 20. The vapor to liquid loading used in scrubbingzone 6 is of course a function not only of the concentration of methanolin the solvent charged to the top of zone 6 via line 11 but also theconcentration of DME in the inflowing vapor stream charged to the bottomof this zone via line 22. As indicated above, the methanolic feed streamto the MTO conversion zone is typically available in a concentrated formof up to 99 mass-% methanol or higher. However, in some cases, where theeconomics dictate the use of a crude methanol stream as the feed streamto the MTO conversion zone, the methanolic content maybe reduced to aslow as 80 mass-% methanol. The methanolic solvent that is charged toscrubbing zone 6 by means of lines 9 and 10 may then have a methanolconcentration of about 80 to 99.9 mass-% methanol and preferably willhave about 95 to 99.9 mass-% methanol when a concentrated methanolicfeed is charged to the MTO conversion zone. The DME concentration in thevapor stream entering zone 6 via line 22 may range from about 0.5 toabout 2.5 mass-% of the total vapor stream. Considering then the rangeof these variable factors, it is appropriate to run scrubbing zone 6 ata solvent-to-vapor mass ratio of about 1:1 to 3:1with the best resultsobtained with a solvent-to-vapor mass ratio of about 1.2:1 to 2.5:1. Theexact choice of the solvent-to-vapor mass ratio used in zone 6 isselected from the ranges specified to preferably eliminate 85 to 99.99wt-% of the DME that enters the zone via line 22. Best practice for amethanolic solvent that contains 99.5 mass-% methanol or higher is touse a solvent-to-vapor loading selected from the ranges specified toremove 95 to 99.9 wt-% of the DME entering the zone via line 22.

At least a portion of the DME-lean light olefin-rich andmethonal-containing overhead vapor stream from primary DME scrubbingzone 6 is then in accordance with the present invention passed via line19 to the lower region of secondary absorption zone 8 in order to removeresidual methanol vapor therefrom. Since the operation of scrubbing zone6 necessarily results in a vapor stream that is saturated with methanolat the conditions prevailing at the top of zone 6 this stream willcontain about 1.5 to 5.5 mass-% methanol. In order to recover this smallamount of methanol dissolved in the overhead vapor stream withdrawn fromzone 6, it is passed via line 20 to the middle region of zone 8 whereinit is scrubbed free of methanol with a portion of the by-product waterstream recovered in oxygenate recovery zone 3 which flows to zone 8 vialines 17 and 18. The details of the operation of zone 8 will bedescribed below.

Returning to the liquid solvent bottom stream recovered from scrubbingzone 6 via line 20, it contains methanol, DME, water and substantial andundesired amounts of ethylene and propylene. In the prior art flowscheme shown in FIG. 2 of U.S. Pat. No. 4,587,373, the liquid productstream from the DME absorber 22 is shown as being passed into aseparator wherein an aqueous phase is separated from a liquidhydrocarbon phase which is then returned to the DME stripping zone 26which operates on the liquid hydrocarbon stream recovered from theprimary separator. It was apparently unappreciated in the prior art thatdespite this phase separation, the methanolic-rich solvent stream stillcontains substantial amounts of dissolved ethylene and propylene which,when they are recycled to the MTO conversion zone via oxygenate stripper18 of the '373 patent, can cause substantial internal recycle of thesevery reactive materials with resulting increase in size of the MTOconversion zone as well as decrease in stability of the catalyst systemdue to undesired polymerization and condensation of these very reactivelight olefin materials. In fact, for a DME scrubbing zone operated asspecified for zone 6 of the present invention, the DME-rich liquidsolvent withdrawn from the bottom of the zone via line 21 even afterphase separation will have dissolved therein approximately 10 to 15mass-% of the ethylene that enters zone 6 via line 22 and about 35 to 50mass-% of the propylene that is charged to this column. The existenceand magnitude of this internal light olefin recycle stream wasapparently not recognized or appreciated in the flow scheme that isshown in FIG. 2 of the '373 patent.

In accordance with the present invention, at least a portion of theliquid solvent bottom stream recovered from scrubbing zone 6 is passedvia line 20 to light olefin stripping zone 7 which is operated at aseverity level sufficient to lift a substantial portion of the ethyleneand propylene contained in this aqueous liquid solvent bottom streamwithout stripping any significant portion of the methanol therefrom toproduce a stripper overhead stream containing DME, ethylene andpropylene and an aqueous bottom stream containing DME, methanol, waterand reduced amounts of light olefin relative to the olefin content ofthe liquid solvent bottom stream charged to this zone 7 by means of line20. It is preferred to operate this light olefin stripping step at aseverity level which is sufficient to lift a substantial portion of thelight olefins while simultaneously maintaining substantially all (i.e.at least 90% or more) of the methanol contained in the input liquidsolvent stream in the liquid solvent bottom stream recovered from zone7. In particular, the severity level is preferrably set such that 95 to99 mass-% of the methanol entering stripping zone 7 via line 20 leavesthis zone in the light olefin-lean bottom liquid stream withdrawntherefrom via line 21. This stripping zone must however be operated at aseverity level which is sufficient to lift a substantial portion of thelight olefins contained in the liquid stream entering the zone via line20. Ordinarily, the severity level measured in terms of temperature,pressure and stripping rate is set to produce an aqueous bottom streamfrom zone 7 containing less than 1 mass-% ethylene with best resultswhen the severity levels are set such that the bottom stream containsless than 0.25 mass-% ethylene. It is to be noted that the operation oflight olefin stripping zone 7 may use a reboiler (not shown) in order togenerate the upflowing vapor streams in a quantity necessary to stripthe light olefins or as is shown in the drawing an optional lightolefin-free gas stream may be injected into the bottom of stripping zone7 via line 11 in order to provide the necessary stripping medium. It isof course within the scope of the present invention to operate strippingzone 7 with a combination of a stripping medium generated via a reboiler(not shown) along with olefin depleted stripping gas which enters zone 7via line 11. The oxygenate-rich and light olefin-lean liquid stream thatis withdrawn from the bottom of light olefin stripping zone 7 via line21 is then recycled via line 9 back to the MTO conversion zone in orderto provide additional reactants for conversion therein.

The light olefin-containing vapor stream withdrawn from stripping zone 7via line 28 will contain minor amounts of DME that co-boil with thepropylene material contained therein and consequently needs to befurthered treated in order to remove this DME. This stream will containan amount of methanol dictated by the methanol liquid-vapor equilibriummaintained at the top of stripping zone 7. This vapor stream willconsequently also contain about 1.5 to 5.5 mass-% methanol along withthe small amount of DME. In accordance with the present invention asubstantial portion of the overhead vapor stream withdrawn from lightolefin stripping zone 7 is passed via line 28 to the lower region ofsecondary DME absorption zone 8 wherein this vapor stream is broughtinto intimate contact with a descending liquid stream containing anaqueous solvent. In view of the facts that the mass flow and DMEconcentration in this vapor stream entering zone 8 via line 28 are muchless than the corresponding numbers for the vapor stream that enters theprimary scrubbing zone 6 via line 22, and that the opportunities forutility savings are much greater, best practice is to use the lowerregion of secondary DME scrubbing zone 8 to absorb DME and methanol fromthis overhead vapor stream. The solvent used in secondary DME scrubbingzone 8 is a portion of its substantially pure by-product water streamrecovered in oxygenate recovery zone 3 and it flows to the upper regionof zone 8 via lines 17 and 18. The contacting means used in zone 8, inorder to promote intimate contact between the ascending vapor streamwith the descending solvent stream, are preferably the same as utilizedin zone 6 and the scrubbing conditions used in zone 8 will include asolvent-to-vapor mass ratio of about 2:1 to 5:1. Recalling that at leasta portion of the DME-lean, light olefin-rich and methanol containingoverhead vapor stream from primary DME scrubbing zone 6 is also chargedto zone 8 via line 19 it is clear that this zone is dual-functional. Itis designed to not only clean-up the residual amount of DME that ispresent in the overhead vapor stream entering this zone via line 28 butalso to remove residual methanol from both vapor streams that arecharged to this zone. According to the present invention thisdual-functionality of zone 8 is enabled by the fact that the methanolcontained in both streams has a high affinity for the aqueous solventand once the methanol content of the aqueous solvent starts to increaseit in turn increases the capacity of this solvent to dissolve DME,thereby generating an effective DME solvent in situ. In order to takemaximum advantage of this finding, it is preferred to separate thepoints of entry of these two vapor stream into zone 8 such that thestripper overhead stream in line 28 that contains DME enters the bottomregion of zone 8 and the primary DME scrubber overhead stream in line 19enters the middle region of zone 8. It is even more preferred that thepoints of entry of these two overhead stream into zone 8 be separated byat least one or more theoretical plate in order to ensure that solvententering the lower region of this zone contain a substantial quantity ofmethanol. Best practice is to separate these two vapor entry points byat least 2 to 5 theorectical plates.

Secondary DME scrubbing zone 8 is operated at scrubbing conditionsselected to produce a DME-lean and methanol-lean overhead vapor productstream containing ethylene and propylene withdrawn by means of line 29and a bottom liquid stream containing DME, methanol and water which ispreferrably recycled via lines 24 and 15 to oxygenate recovery zone 3wherein the oxygenates contained therein are stripped and recycled tozone 1 via lines 16 and 9 to provide additional quantities of reactantthereto. It is a preferred practice to pass the DME-lean overheadhydrocarbon vapor product stream withdrawn from zone 8 via line 29 todownstream facilities (not shown) in order to further purify this streamand recover polymer-grade ethylenes and propylenes.

The following examples are presented in order to facilitate furtherunderstanding of the present invention in reference to a controlexample. They are however presented for purposes of illustration ratherthan limitation. These examples will contrast the results using theproduct recovery scheme of the present invention with those obtained viathe product recovery scheme taught in the prior art. The productrecovery scheme of the present invention is shown in FIG. 1 and theproduct recovery scheme of the prior art is shown in FIG. 2. Acomparison between the two figures will show that, to the extentpossible, common elements between the two product work-up flow schemesuse identical numbers in order to make direct comparison easier. In bothof the examples, the MTO conversion zone is run in a fluidized bed modeof operation with a SAPO-34 type of catalyst. The SAPO-34 catalyst isprepared in accordance with the teachings of U.S. Pat. No. 5,191,141 andis used in a particle size appropriate for fluidization (i.e. averageparticle size diameter is about 70 to 80 microns). The catalyst particleformulation used in both of these examples is based on a 40 mass-%SAPO-34 molecular sieve bound with 60 mass-% of inert filler/bindermaterials.

The feed stream charged to the MTO conversion zone 1 via line 9 in bothcases comprises a mixture of methanol and water containing 99.85 wt-%methanol. No diluent is used in either of these examples but substantialamounts of a steam diluent is autogenously generated due to the veryrapid kinetics associated with the formation of intermediate DME.

The operating conditions used in zone 1 in both cases are as follows:(1) feed stream entry temperature of about 100° C. (212° F.) and aneffluent exit temperature of about 475° C. (887° F.); (2) a pressure ofabout 239 kPa (34.7 psi); (3) a WHSV of 2.4 hr⁻¹ based on mass of totalcatalyst with the catalyst being recirculated after product separationin a conventional manner primarily via internal or external catalystrecycle means; and (4) a catalyst draw-off rate for regeneration fromthe circulating inventory of catalyst in zone 1 in amounts sufficient tomaintain an average catalyst coke content of the circulating inventoryof catalyst in the range of about 2 to 5 mass-%. The drawn-off catalystis oxidatively regenerated in an associated conventional regenerator(not shown in either of the attached drawings) to reduce coke level toabout 1 mass-% or less and the resulting regenerated catalyst isrecirculated to the MTO conversion zone in order to maintain thespecified average coke level on the circulating catalyst inventory.

Operation of zone 1 in both of these examples at these conditions withthe specified catalyst system and the methanol feedstock results in 99.5mass-% conversion based on methanol disappearance and the yieldstructure shown in Table 1 based on an appropriate carbon balance.

Because methanol contains about 56 mass-% bound water, water is a verysubstantial by-product of the MTO reaction that occurs in zone 1 andsubstantial amounts of water must be condensed out of the effluentstream and separated from the hydrocarbon products of the MTO reactionzone in order to prepare the portion of effluent stream which is chargedto the product recovery flow scheme in both of these examples.

TABLE 1 Yields from MTO Conversion Zone Selectivity (Mass-% of MethanolComponent Equivalent) Methane 0.85 Ethylene 33.50 Ethane 0.65 Propylene44.50 Propane 0.30 Butylene 9.5 Butane 0.01 C₅₊ 5.00 Coke 2.85 UnreactedMethanol 0.50 DME 1.05 Others (H₂, CO, CO₂, Dienes) 1.29 TOTAL 100.0

The effluent stream exiting MTO conversion zone 1 via line 12 can besubjected to conventional cooling procedures such as feed/effluent heatexchange (not shown) and is charged to quench zone 2 wherein itundergoes further cooling in order to condense a substantial amount ofthe by-product water contained therein. Quench zone 2 operates aspreviously described with a circulating quenching medium which isprimarily water and countercurrently contacts the hot, steam-richeffluent stream from zone 1 in a manner designed to condense a majorportion of the by-product water from the MTO conversion step and to dropthe temperature of this effluent stream such that the hydrocarbonportion leaves zone 2 via line 13 at a temperature of about 40° C. (104°F.) and a pressure of about 200 kPa (29 psi). In both of the examples, aportion of the circulating aqueous medium in line 14 is drawn off vialine 15 and passed to oxygenate recovery zone 3. Zone 3 operates atconventional oxygenate stripping conditions in order to stripsubstantially all oxygenates which are then recycled to MTO conversionzone 1 via lines 16 and 9. These oxygenate materials accumulate in thiscirculating quenching medium due to their high solubility therein. Anaqueous stream withdrawn from the lower region of oxygenate recoveryzone 3 via line 17 is substantially pure water with very minor amountsof contaminants dissolved therein.

The hydrocarbon and oxygenate-containing vapor overhead from the top ofquench zone 2 is passed via line 12 into the primary product separationzone 4. During the passage of this vapor stream from zone 2 to zone 4,it is passed through a series of suction drums, compressors and coolers(not shown in the attached drawing) to increase its pressure to about2069 kPa (300 psi) and as a result, it enters primary product separatorzone 4 at a temperature of about 38° C. (100° F.) and is held at theseconditions in zone 4 in order to produce a three-phase separationwhereby a hydrocarbon-rich vapor phase is withdrawn from zone 4 via line22, a heavy hydrocarbon liquid phase is withdrawn from the zone via line25 and an aqueous phase is withdrawn therefrom via line 23. In FIG. 1used in Example 1, the aqueous phase from zone 4 is withdrawn therefromvia line 23 and passed via lines 23, 24 and 15 into oxygenate recoveryzone 3. In the flow scheme shown in FIG. 2 used in Example 2, thisaqueous stream from the primary product separator is also passed to zone3 except in this case, it travels via lines 23, 20 and 15 to oxygenaterecovery zone 3.

In both Examples 1 and 2, the hydrocarbon liquid phase that is withdrawnfrom primary product separating zone 4 via line 25 is passed to DMEstripping zone 5 which operates at a pressure at the top of the stripperwhere the overhead is withdrawn which is 172 kPa (25 psi) less than thepressure maintained in zone 4. DME stripping zone 5 uses a reboiler (notshown in either drawing) in order to generate upflowing vapors that inboth cases lift DME and light olefins dissolved in the hydrocarbon inputstream into the overhead stream which is withdrawn from this zone vialine 26 and flows to the junction with line 22 wherein it is combinedwith the overhead from zone 4. The resulting mixture is then passed intoprimary DME scrubbing zone 6 via line 22. A liquid hydrocarbon streamwhich is rich in C₄ ⁺ material is withdrawn from the bottom of zone 5 bymeans of line 27 in both Examples 1 and 2. This bottom stream comprisesthe C₄ and C₅ olefin products of the MTO conversion zone along withvarious other heavier hydrocarbon by-products. This heavy hydrocarbonproduct stream is withdrawn via line 27 and passed to further heavyhydrocarbon treatment facilities that are located downstream from theMTO process.

In the flow scheme of the present invention used in Example 1 as well asthe flow scheme of the prior art used in Example 2, zone 5 provides anadditional means of DME recovery and also light olefin recovery. Theoverhead vapor stream that is separated in primary product separator 4is combined with the overhead vapor stream from stripping zone 5 at thejunction of lines 22 and 26 in both cases and the resulting combinedvapor stream flows via line 22 to the lower region of primary DMEabsorption zone 6. It is to be recognized that because of pressuredifferences between the vapor streams present in line 22 and line 26, acompressor may be needed in line 26 to ensure that the flow of the DMEoverhead stream from zone 5 is as described.

The product recovery scheme of the present invention (shown in FIG. 1)and that of the prior art (shown in FIG. 2) both utilize amethanolic-rich solvent in a scrubbing zone 6 in order to remove DMEfrom the light hydrocarbon products of the MTO conversion reaction. Themethanolic solvent is obtained in both cases as a portion of themethanolic feed stream which enters the process via line 9. The portionthat is used in scrubbing zone 6 is diverted in line 10 and passes intothe upper region of scrubbing zone 6 wherein it flows in countercurrentfashion against the upflowing combined hydrocarbon-containing vaporstream which enters the lower region of zone 6 via line 22. The amountof methanolic solvent that is diverted via lines 9 and 10 for use inscrubbing DME and other oxygenates from the light hydrocarbon vaporsinjected into the bottom of zone 6 via line 22 is about 50 to 100 mass-%of the total feed that is charged to the MTO conversion process via line9. In both cases, zone 6 is operated at DME scrubbing conditionsincluding a temperature of 54.4° C. (130° F.), a pressure of 2020 kPa(293 psi) and a solvent-to-vapor mass ratio of about 1.32:1 to produce aDME-lean and light hydrocarbon-rich overhead vapor stream, which in bothcases exits zone 6 via line 19 and a DME-rich methanolic solvent liquidstream which exits zone 6 via line 20. The temperature and pressure ofzone 6 as recited above are measured at the point of withdrawal of theoverhead vapor stream.

Having completed a description of the common elements of the flow schemethat is used in both Examples, the following two examples are structuredto make manifest the differences between the operation of the flowscheme of FIG. 1 an embodiment of the present invention used in Example1 and of the prior art scheme FIG. 2 used in Example 2.

EXAMPLE 1 Present Invention

The product recovery flow scheme utilized in this example is shown inFIG. 1 in this embodiment, primary DME scrubbing zone 6 operates asdescribed above in the discussion of the common elements between the twoflow schemes to produce an overhead vapor stream withdrawn by means ofline 19 which is DME-lean and C₂ and C₃ olefin-rich. Because this lightolefin-rich vapor stream also contains an amount of methanol dictated bythe vapor-liquid equilibrium conditions maintained at the top of zone 6,it is charged via line 19 to the middle region of secondary DME recoveryzone 8 where this overhead vapor stream from zone 6 countercurrentlycontacts an aqueous solvent stream under conditions selected to scrubsubstantially all of the methanol from this vapor stream. The aqueoussolvent used in secondary DME recovery zone 8 is provided to this zonevia lines 17 and 18 and is a portion of the by-product water stream thatis produced in zone 3. In accordance with the present invention zone 8also performs the final polishing of the overhead vapor stream producedin light olefin stripping zone 7 which flows to zone 8 by means of line28. The entry point of this second overhead stream is in the lowerregion of zone 8 since this second overhead stream contains not onlymethanol but also a minor amount of DME that co-boils with the propylenecontained therein as explained previously. In accordance with apreferred embodiment of the present invention the entry points into zone8 for these two overhead vapor streams are separated by a verticaldistance which corresponds to at least one or more theoritical plates.This separation of entry points for the two overhead streams that arescrubbed in zone 8 facilitate the recovery of both DME and methanol inthe bottom region of this zone since the descending aqueous solvententering the lower region of zone 8 will have a significant methanolcontent acquired by absorption from the overhead vapor stream charged tothis zone via line 19. Since methanol has a high affinity for DME, theaqueous solvent containing methanol entering the lower region of zone 8(i.e. below the entry point of line 19) is a highly effective solventfor DME and prevents any significant DME contamination of the lightolefin product stream withdrawn from zone 8 via line 29. Zone 8 containssuitable contacting means to promote the interaction of the aqueoussolvent stream injected via line 18 with the two upflowing vapor streamsone of which enters the middle region of zone 8 via line 18 and one ofwhich enters the bottom of zone 8 via line 28. Zone 8 operates at atemperature of about 25° C. (77° F.), a pressure of about 1813 kPa (263psi) and a solvent-to-total-vapor mass loading ratio of about 5:1. Zone8 therefore operates to eliminate substantially all DME and methanolfrom the light olefin product stream of the present invention which isproduced as an overhead stream and is withdrawn from this zone via line29 and constitutes the principal light olefin product stream of theproduct recovery scheme of the present invention. The DME andmethanol-containing aqueous solvent stream that is withdrawn from thebottom of zone 8 is recycled to zone 3 via lines 24 and 15 in order toeventually return the DME and methanol values contained therein to zone1 via lines 16 and 9.

Returning to the operation of zone 6, the methanolic solvent streamwhich is rich in DME that is withdrawn from zone 6 via line 20 is inaccordance with the present invention charged to light olefin strippingzone 7 and enters this zone at or below the mid-point thereof. Lightolefin stripping zone 7 operates in accordance with the presentinvention at a severity level which is sufficient to strip a substantialportion of the C₂ and C₃ olefins contained in the DME-rich solventstream that is charged to this zone via line 20 but insufficient toremove a significant portion of the methanol that is contained in thissolvent stream. In other words, the severity used in zone 7 issubstantially less than the severity level that is used in oxygenaterecovery zone 3 where it is desired to lift substantially all oxygenatesinto the overhead vapor stream. Zone 7, as previously explained, canoperate with a stripping gas medium which is olefin-lean and rich inmethane which can enter zone 7 via line 11. Alternatively, zone 7 canoperate with a reboiler (not shown) and autogenously generate upflowingvapors by reboiling a portion of the light olefin-depleted methanolsolvent that is withdrawn from the bottom of this zone via line 21. Forpurposes of this example, line 11 is blocked off and a reboiler systemis used to generate upflowing vapors with a suitable heat recovery means(not shown) located in line 28 to minimize the utilities associated withthis reboiling operation. In this example, zone 7 is operated atconditions which are sufficient to strip substantially all of theethylene contained in the DME-rich solvent that is charged to this zone.These conditions include a pressure of 1827 kPa (265 psi) and atemperature of 76.7° C. (170° F.) which conditions are measured at thepoint of withdrawal of the overhead vapor stream from zone 7 via line28. Under these conditions, zone 7 operates to remove 100% of theethylene present in the DME-rich methanolic solvent charged thereto vialine 20 and 51.2% of the propylene which is dissolved in the DME-richmethanolic solvent. The resulting light olefin-rich vapor overheadstream which is withdrawn from zone 7 via line 28 is unfortunatelycontaminated with a minor amount of DME due to the strong affinitybetween DME and propylene. It is in accordance with the presentinvention this overhead vapor stream is passed via line 28 to the lowerregion of secondary DME scrubbing zone 8 in order to scrub methanol andthe contaminating DME therefrom. The bottoms stream from zone 7 iswithdrawn therefrom via line 21 and is a light olefin-lean, DME-richmethanolic stream which is returned to MTO conversion zone 1 via lines21 and 9.

The calculated separation recovery efficiencies associated with theembodiment of the present invention described in this example arepresented in Table 2 in tabular form wherein the first column shows theprincipal products and methanol (includes both unreacted and solventmethanol), the second column shows the percentage of this product thatis recovered in the recycled liquid solvent stream 21 and in therecycled liquid aqueous stream 24 and the third column presents thepercentage of the product that is recovered in the light olefin productstream withdrawn from the instant product recovery method via line 29.By reference to the second line of Table 2, it can be shown that thepresent invention drives the ethylene recovery efficiency of the productrecovery flow scheme to 100% which stands in sharp contrast to the priorart recovery rate of 87.71% which is hereinafter shown in Table 3.

TABLE 2 Separation and Recovery Efficiencies* for Product Recovery FlowScheme of Present Invention MTO Reactant and % Recovered RecoveredReaction Products in Recycle Streams in Product Stream Methane 0 100Ethylene 0 100 Ethane 0 100 Propylene 20.0 80.0 Propane 19.13 80.87 DME99.20 0.80 Methanol** 99.90 0.10 *Measured in mass-% of the MTO reactionproduct or reactant recovered on a per pass basis. **Includes bothunreacted methanol present in effluent stream 12 as well as methanolsolvent directed to scrubbing zone 6 via lines 9 and 10.With reference to the fourth line of Table 2, it can be seen that thepropylene recovery rate increases to 80% which stands in contrast to the59.72% recovery rate associated with the prior art flow schemehereinafter exemplified in the control Example 2. It is to be noted thatthis increase in recovery efficiencies for the desired light olefin wasnot accompanied by any sacrifice in the efficiency of DME recovery whichis clearly demonstrated by comparing the results of the DME recoveryefficiencies from Tables 2 and 3. The methanol recovery numbers given inthe last line of Table 2 shows the substantial improvement in recoveryof this reactant that is enabled by the two stage scrubbing procedure ofthe present invention.

EXAMPLE 2 Control

With reference now to the details associated with operations of DMEscrubbing zone 6 in FIG. 2 in accordance with the teachings of U.S. Pat.No. 4,587,373, it can be seen that the DME-rich methanolic solvent thatis withdrawn from the bottom of zone 6 is passed via line 20 and line 15to oxygenate recovery zone 3 in order to strip both the recovered DMEand the methanolic solvent from any water that is present in this streamand to recycle the resulting DME intermediate and unreacted methanol tozone 1 via line 16. It is to be noted that the DME-rich methanolicsolvent that is withdrawn from zone 6 via line 20 can be passed into aphase separator (not shown) in a conventional manner in order to allowrecovery of any relatively high boiling hydrocarbons (i.e. C₄+ material)that condense in zone 6 due to the conditions maintained therein. TheDME-lean light olefin-containing vapor product stream is then withdrawnfrom the upper region of scrubbing zone 6 via line 19 and passed tofurther downstream processing not a part of the MTO process of the '373patent. This product stream also contains a significant amount ofmethanol dictated by the vapor-liquid equilibrium conditions maintainedat the top of zone 6. In this case these conditions results in theoverhead vapor stream containing 2.15 mass-% of the total methanolcharged to zone 6 in the form of solvent methanol entering via line 10and unreacted methanol via line 22.

FIG. 2 is intended to represent the flow scheme that is shown in FIG. 2of U.S. Pat. No. 4,587,373. In FIG. 2 of the '373 patent, zone 12corresponds to zone 1 of the attached FIG. 2 and is the MTO conversionzone. Quenching zone 2 is functionally represented by cooling zone 14 ofthe '373 patent. The oxygenate recovery zone 3 of the attached FIG. 2corresponds to oxygenate stripper 18 of the '373 patent. Primary productseparator 4 corresponds to product separator 16 of the '373 patent. DMEstripping zone 5 is represented in FIG. 2 of the '373 patent bystabilizer tower 26 and DME scrubbing zone 6 is represented by DMEabsorber 22.

By utilizing the operating conditions hereinbefore specified, the MTOconversion yields shown in Table 1 and the known affinity for DMEexhibited by a rich methanolic solvent, the separation and recoveryefficiencies associated with the prior art product recovery flow schemewere calculated utilizing a conventional chemical process flow schemesimulation program (which was also used in the calculation presented inTable 2). The results of these calculations are presented in Table 3 interms of separation and recovery efficiencies for methanol and thevarious key products of the MTO conversion zone.

TABLE 3 Separation and Recovery Efficiencies* for Prior Art ProductRecovery Flow Scheme MTO Reactant and % Recovered % Recovered ReactionProduct in Recycle Stream in Product Stream Methane 3.43 96.57 Ethylene12.29 87.71 Ethane 11.77 88.23 Propylene 40.28 59.72 Propane 40.99 59.01DME 99.0 1.0 Methanol** 97.85 2.15 *Measured in mass-% of the MTOreaction product or methanol recovered on a per pass basis. **Includesboth unreacted methanol and solvent methanol.

The efficiencies recited in Table 3 are measured in terms of mass-% ofmethanol entering zone 6 and of the particular MTO reaction product thatis specified in the first column that is recovered in the productseparation system on a per pass basis. For example, in the case ofethylene, the results show that 87.71 mass-% of the ethylene produced inMTO conversion zone 1 is recovered in the overhead stream 19 from DMEscrubbing zone 6 which in the light olefin product stream for the '373patent. In contrast, the remaining portion of the ethylene is dissolvedin the methanolic-rich solvent withdrawn from the bottom of scrubbingzone 6 via line 20 and recycled to MTO conversion zone 1 via lines 22,15 to zone 3 and then via lines 16 and 9. The results for propylene areeven more interesting in that 59.72 mass-% of the propylene produced inzone 1 exits the process via line 19 as part of the light olefin productstream of the product recovery system illustrated in FIG. 2. Quitesurprisingly however, a substantial portion of the propylene finds itsway back to MTO conversion zone 1 because of its high solubility in themethanolic solvent used in DME scrubbing zone 6. The recycled propylenetravels via lines 20 and 15 into zone 3 wherein it is stripped into theoverhead stream exiting zone 3 via line 16 wherein it is ultimatelyreturned to MTO conversion zone 1 via lines 16 and 9.

The principal lesson to be learned from an examination of the results ofthe calculations presented in Table 2 is that the use of a methanolicsolvent in DME scrubbing zone 6 has the unintended consequence ofdragging substantial amounts of C₂ and C₃ olefins into an internalrecycle stream where they are returned to MTO conversion zone 1 andultimately creates a light olefin internal circuit in the flow schemethat builds up the concentration of these materials in stream 12 untilthe amounts that exit the system via line 19 balance the amounts thatare produced in MTO reaction zone 1. This light olefin recycle circuitthat therefore occurs in the flow scheme of the '373 patent is anunintended consequence of using methanol as the solvent in the scrubbingzone in order to take advantage of methanol's well-known high affinityfor DME.

By contrasting the results shown in Table 2 with those presented inTable 3, it is evident that the instant invention eliminates asignificant loss of methanol in the light olefin product stream andenables a substantial reduction in the rate of recycle of light olefinsthat is necessary to recover 100% of the desired light olefin productswhich are produced in MTO conversion zone 1. The degree of improvementassociated with the embodiment of the present invention shown in Example1 over the prior art flow scheme exemplified in Example 2 is highlightedin Table 4 which presents the recycle ratio required to recover 100% ofthe two desired light olefin products of the MTO reaction step in termsof the recycle ratio necessary for each of these products to force 100%of the product from zone 1 to exit the system via line 29 in the case ofFIG. 1 and via line 19 in the case of FIG. 2.

TABLE 4 Recycle Ratio* Required to Recover 100% of the C₂ and C₃ OlefinProducts of MTO Reaction Step** Prior Art Present Invention Product FlowScheme Flow Scheme Improvement Ethylene 1.14 1.0 12.3% Propylene 1.6941.25 26.2% *(Recycle + Feed)/Feed **Basis is amount of product that mustbe recycled to result in 100% recovery of that product on a per passbasis.

By reference to the first line of Table 4, the instant invention showsan improvement in the recycle ratio parameter of 12.3% for ethylene. Theresults are even more dramatic with respect to the propylene product ofthe MTO conversion reaction. As is shown in line 2 of Table 4, theinstant invention results in a 26.2% improvement in the recycle ratio.The present invention acts dramatically and convincingly tosubstantially lower the recycle ratio required in order to recover thehighly desired light olefin products. This diminishment of this recycleratio obviously results in a substantial shrinkage of the size of MTOconversion zone 1 due to the diminished amount of ethylene and propylenethat must be recycled through this zone. In addition, the presentinvention acts to keep substantial amounts of highly reactive lightolefins out of the MTO conversion zone, thereby enhancing the stabilityof the catalyst system contained therein by eliminating substantialamounts of potential coke precursors.

1. A method of selective recovery of a DME-containing recycle stream anda DME-lean, methanol-lean and light olefin-rich product stream from theeffluent stream from a MTO conversion zone wherein the effluent streamcontains water, methanol, DME, ethylene, propylene and C₄ to C₆ olefins,which method comprises the steps of: (a) cooling and separating at leasta portion of the effluent stream into an aqueous liquid streamcontaining methanol and DME, a hydrocarbon liquid stream containingmethanol, DME and C₄ to C₆ olefins and a hydrocarbon vapor streamcontaining DME, methanol, ethylene and propylene; (b) stripping DME fromat least a portion of the liquid hydrocarbon stream separated in step(a) in a DME stripping zone operated at stripping conditions effectiveto produce an overhead vapor stream containing DME, methanol, ethyleneand propylene and a liquid hydrocarbon bottom stream containing C₄ to C₆olefins; (c) combining at least a portion of the hydrocarbon vaporstream separated in step (a) with at least a portion of the overheadvapor stream produced in step (b) to form a DME-rich light hydrocarbonvapor stream; (d) charging the resulting DME-rich light hydrocarbonvapor stream to a primary DME absorption zone and therein contactingthis vapor stream with a DME selective solvent containing methanol atscrubbing conditions effective to produce (1) a liquid solvent bottomstream containing methanol, DME, water and substantial and undesiredamounts of ethylene and propylene and (2) a light olefin-rich, DME-leanoverhead vapor stream containing methanol; (e) passing at least aportion of the liquid bottom stream recovered from step (d) to a lightolefin stripping zone operated at stripping conditions effective tostrip at least a substantial portion of the ethylene and propylenecontained in the liquid bottom stream without stripping any significantportion of the methanol therefrom to produce a stripper overhead streamcontaining DME, methanol, ethylene and propylene and a liquid bottomstream containing DME, methanol, water and reduced amounts of lightolefins relative to the light olefin content of the liquid solventbottom stream charged to this step; (f) recycling at least a portion ofthe liquid bottom stream recovered from step (e) to the MTO conversionzone thereby selectively providing additional oxygenate reactantsthereto; and (g) charging at least a portion of the stripper overheadstream from step (e) and at least a portion of the light olefin-rich,DME-lean overhead vapor stream from step (d) to a secondary DMEabsorption zone wherein these streams are countercurrently contactedwith an aqueous solvent at scrubbing conditions selected to produce aDME-lean and methanol-lean overhead vapor product stream rich inethylene and propylene and a bottom liquid stream containing DME,methanol and the aqueous solvent.
 2. The method as defined in claim 1wherein at least a portion of the aqueous liquid stream separated instep (a) and at least a portion of the bottom liquid stream from step(g) are combined and charged to an oxygenate recovery zone which isoperated at oxygenate stripping conditions effective to produce anoverhead vapor stream rich in DME and methanol and an aqueous bottomstream which is substantially free of oxygenates and wherein at least aportion of the overhead vapor stream is recycled to the MTO conversionzone thereby providing additional oxygenate reactants thereto.
 3. Themethod as defined in claim 2 wherein the aqueous solvent used in step(g) is at least a portion of the aqueous bottom stream recovered fromthe oxygenate recovery zone.
 4. The method as defined in claim 2 whereinthe stripper overhead stream from step (e) that is charged to step (g)enters the secondary DME absorption zone in the bottom region thereofand wherein the light olefin-rich, DME-lean overhead vapor stream fromstep (d) that is charged to step (g) enters the secondary DME absorptionzone at least one or more theoretical plates higher in the zone than thepoint of entry of the stripper overhead stream.
 5. The method as definedin claim 4 wherein the points of entry of the two overhead streams thatare charged to the secondary DME absorption zone are separated by atleast 2 to 5 theoretical plates.
 6. The method as defined in claim 1wherein the DME-selective solvent used in step (d) contains about 80 to99.99 mass-% methanol.
 7. The method as defined in claim 6 wherein theDME-selective solvent used in step (d) is about 95 to 99.99 mass-%methanol.
 8. The method as defined in claim 1 wherein the scrubbingconditions utilized in step (d) include a solvent-to-vapor mass ratio ofabout 1:1 to 3:1.
 9. The method as defined in claim 8 wherein thescrubbing conditions used in step (d) include a solvent-to-vapor massratio of about 1.2:1 to 2.5:1.
 10. The method as defined in claim 1wherein the secondary DME absorption zone used in step (g) is operatedat scrubbing conditions including a solvent-to-vapor mass ratio of about2:1 to 5:1.
 11. The method as defined in claim 1 wherein theDME-selective solvent charged to step (d) is a portion of the methanolfeed stream to the MTO conversion zone.
 12. The method as defined inclaim 1 wherein the stripping conditions used in step (e) are set toproduce a liquid bottom stream containing less than about 1 mass-%ethylene.
 13. The method as defined in claim 12 wherein the strippingconditions used in step (e) are set to produce a liquid bottom streamcontaining less than about 0.25 mass-% ethylene.
 14. A method ofselective recovery of a DME-containing recycle stream and a DME-lean,methanol-lean and light olefin-rich product stream from the effluentstream from a MTO conversion zone wherein the effluent stream containswater, methanol, DME, ethylene, propylene and C₄ to C₆ olefins, whichmethod comprises the steps of: (a) cooling and separating at least aportion of the effluent stream into an aqueous liquid stream containingmethanol and DME, a hydrocarbon liquid stream containing methanol, DMEand C₄ to C₆ olefins and a hydrocarbon vapor stream containing DME,methanol, ethylene and propylene; (b) stripping DME from at least aportion of the liquid hydrocarbon stream separated in step (a) in a DMEstripping zone operated at stripping conditions effective to produce anoverhead vapor stream containing DME, methanol, ethylene and propyleneand a liquid hydrocarbon bottom stream containing C₄ to C₆ olefins; (c)combining at least a portion of the hydrocarbon vapor stream separatedin step (a) with at least a portion of the overhead vapor streamproduced in step (b) to form a DME-rich light hydrocarbon vapor stream;(d) charging the resulting DME-rich light hydrocarbon vapor stream to aprimary DME absorption zone and therein contacting this vapor streamwith a DME selective solvent containing methanol at scrubbing conditionseffective to produce: (1) a liquid solvent bottom stream containingmethanol, DME, water and substantial and undesired amounts of ethyleneand propylene and (2) a light olefin-rich, DME-lean overhead vaporstream containing methanol; (e) passing at least a portion of the liquidsolvent bottom stream recovered from step (d) to a light olefinstripping zone operated at stripping conditions effective to strip atleast a substantial portion of the ethylene and propylene contained inthe liquid solvent bottom stream without stripping any significantportion of the methanol contained therein to produce a stripper overheadstream containing DME, methanol, ethylene and propylene and a liquidsolvent bottom stream containing DME, methanol, water and reducedamounts of light olefins relative to the input olefin content of theliquid solvent bottom stream charged to this step; (f) charging at leasta portion of the stripper overhead stream from step (e) and at least aportion of the light olefin-rich, DME-lean overhead vapor stream fromstep (d) to a secondary DME absorption zone wherein these streams arecountercurrently contacted with an aqueous solvent at scrubbingconditions selected to produce a DME-lean and methanol-lean overheadvapor product stream rich in ethylene and propylene and a bottom liquidstream containing DME, methanol and water; (g) passing at least aportion of the aqueous liquid stream separated in step (a) and at leasta portion of the bottom liquid stream produced in step (f) to anoxygenate recovery zone and therein subjecting these streams tooxygenate stripping conditions effective to produce an overhead vaporstream rich in DME and methanol and at aqueous bottom stream which issubstantially free of oxygenates; (h) recycling at least a portion ofthe liquid solvent bottom streams recovered from step (e) and at least aportion of the overhead vapor stream produced in step (g) to the MTOconversion zone thereby selectively providing additional oxygenatereactants thereto; and (i) passing at least a portion of the aqueousbottom produced in step (g) to step (f) to provide at least a portion ofthe aqueous solvent used therein.
 15. The method as defined in claim 14wherein the DME selective solvent charged to step (d) is a portion ofthe methanol feed stream to the MTO conversion zone.
 16. The method asdefined in claim 14 wherein the stripper overhead stream from step (e)charged to the secondary DME absorption zone enter this zonesubstantially below the point of entry of the light olefin-rich,DME-lean overhead vapor stream from step (d).
 17. The method as definedin claim 16 wherein the points of entry of the two overhead streamscharged to the secondary DME absorption zone are separated by at leastone or more theoretical plates.